How can I fix this normal map shading issue?
Your problem is the UV unwrap. If you look at your texture in textured view, you will see a zig-zag seam in the mapping. Your staff, unfortunately, has a very irregular topology so it will be hard to unwrap properly. 
2 solutions. The easy one first, but it will only work from certain angles.

Position your camera, look through it at the model, select the model, go into edit mode, press U and select project from view. Quick and dirty

Or use projection paint to manually clone the texture onto your object. In edit mode press U -> smart UV project. And then do what's called projection painting, there are lots of tuts about that, e.g. here:
https://www.youtube.com/watch?v=eQy5jn9dWrs